Such a method and such a control device are each discussed in DE 103 05 656 A1. In this context, characteristic quantities are ascertained from the signal of the structure-borne noise sensor, the structure-borne noise signal first being filtered in at least two crankshaft angular ranges. A characteristic quantity may be ascertained, for each angular range, which characterizes the intensity of the sound emission in that angular range. The characteristic quantities ascertained characterize certain events and points in time in the work cycle of the internal combustion engine. In particular, in a preinjection, there exists a simple relationship between the intensity of the sound emission connected with it and the injected fuel quantity, so that the injected fuel quantity of a preinjection is able to be ascertained from the structure-borne noise signal according to the method broadly presented in DE 103 05 656 A1.
In a metering of fuel into the combustion chambers of an internal combustion engine taking place via injectors, the injectors are activated by injection pulse widths which open a flow cross section of the injectors for the duration of the injection pulse width.
Although the injection pulse width can be specified very accurately, the fuel quantity injected in each case in this context is a function of, among other things, the injection pressure and of properties of the injector itself, for instance, of a response delay at which the injector reacts to an injection pulse. These properties may be scattered from injector to injector, so that the fuel quantities respectively injected are also subject to undesired scatter. As a result, for example, the exhaust gas emissions of the internal combustion engine and/or the running properties of the internal combustion engine are influenced negatively. This applies particularly to the so-called main injection, because in it the greatest portion of the fuel quantity to be injected for a combustion process is metered. This brings about an interest in compensating for the scattering mentioned, which come up in deviations of an actually injected fuel quantity from a setpoint value for the fuel quantity to be injected. These deviations will also be designated below as quantity errors.
Therefore, there is a need for compensation or correction of the quantity errors in both the preinjections and the main injections. As was mentioned before, a comparatively simple relationship between the relatively small preinjection quantity and the structure-borne noise signal has turned out to exist, during testing. In other words: The relatively small preinjection quantities, and thus also their quantity errors, may be detected quite well from the structure-borne noise signal. However, it also turned out that this does not apply for the main injection quantities, which are greater in order of magnitude by a factor of 10 than the preinjection quantities. That is, the quantity errors of the main injection quantities cannot be detected directly with sufficient accuracy from the structure-borne noise signal. This applies analogously for all additional injections via which large fuel quantities are metered compared to the preinjection. The structure-borne noise signal is particularly suitable for this, but other output signals of other sensors, that indicate that a combustion has taken place, can also be used.
In light of this background, the exemplary embodiments and/or the exemplary methods of the present invention provides, for a method of the type mentioned at the outset, that a correction value is formed as a function of the quantity error of the preinjection, and at least one of the additional injections is corrected using the correction value. Correspondingly, for a control device of the type mentioned at the outset, it is provided that the control device forms a correction value as a function of the quantity error of the preinjection, and corrects at least one of the additional injections using the correction value.